Systems that are exposed to the harsh environments of space must be able to withstand extreme conditions operate correctly without losing any of their functions and capabilities. Specifically, space based systems must operate in an environment in which radiation has an adverse impact on integrated circuit operation.
Four potential problems may arise due to the effects of radiation. Three of these problems can permanently damage integrated circuits, while the fourth problem may be remedied.
First, total dose radiation caused by the cumulative effects of particles striking an integrated circuit can permanently damage that integrated circuit. Second, the radiation dose rate caused by burst radiation may permanently damage the integrated circuit. Third, displacement damage resulting from nuclear interactions, namely scattering that causes semiconductor defects, can cause permanent damage to integrated circuits. Fourth, single event upsets (SEU or SEUs) can cause a change of state (usually in a memory bit). SEUs do not, however, cause permanent damage.
SEUs occur when energetic particles deposit a charge into memory circuits, causing stored data to change state (i.e., from a “1” to a “0,” or vice versa). As circuits shrink and transistor volumes become smaller, the total charge needed to cause an upset in a circuit element decreases. Thus, even protons moving through the circuit may deposit sufficient charge to disrupt sensitive locations.
The most common approach for correcting errors is to use triplicated-redundant information storage or error-checking circuitry. For example, a technique known as “voting logic” can be used to catch and correct potential errors in latches. With this technique, a single latch does not effect a change in bit state; rather, several identical latches are queried, and the state will only change if the majority of latches are in agreement. Thus, a single latch error will be “voted away” by the others.
The majority voting may be performed internal to a device and/or external to a device. Typically, the internal majority voting is simpler and can be executed at a low-level where the potential errors occur. The external majority voting can correct major errors that cannot typically be corrected by the internal majority voting. For example, a Single Event Functional Interrupt (SEFI), such as, device shutdown, loss of lock, loss of input/output or other disabling events are typically not corrected by the internal majority voting.
Generally, whether the majority voting is performed internal or external to the device, voting circuits require the implementation of an on-board low skew timing circuit, resulting in minimal clock skew between all the components. The voting logic compares for multiple clock differences between data streams from each of the triple-redundant data storage to determine if an error has occurred. In the event of an error, the voting logic must separately command the re-synchronization of the system.
As circuit speed increases, the margins necessary for low skew timing circuits decrease substantially. As a result, critical timing adjustments are required. These tight timing requirements limit the number of circuit boards that can be utilized in the system. In addition, the voter method of comparing multiple clock differences requires substantial internal circuitry for detecting voting errors, and for resynchronizing the system.
The present system and method solves these and other problems by providing, self-timing automatic synchronization. The present system also allows remote synchronization, through network based systems, without being affected by the number of boards or modules used in that system.